we're talking about art here bud. I know they seem similar because they're made in the same medium, and are both visual, but you have to understand the difference if you're going to critique one or the other.
making art is a process that is always evolving. there are no finished pieces in art, only attempts at projecting an idea in as many different means as possible. art is an inherently conceptual task, deals with symbols and the abstract. it is the world of the unknown, the world of fear, and the world of uncertainty. to make art you must put yourself in a life or death situation, and THEN attempt to create. your life is dependent on your creation, and it's contribution to our collective perception. these are the pressures of an artist.
illustration, on the other hand, is a craft that has objectives that are met. you 'finish' and illustration, and it is judged based on it's 'perfection' and efficiency, not necessarily it's conceptual ingenuity (although there are often 'concept' illustrators that 'illustrate' puzzles and riddles and clever jokes, this is not to be confused with the task of an artist). illustrators are not necessarily thinkers, they are executors of a craft to fulfill the objective that is laid out to them by another. illustrators have a limited creative autonomy when making.
illustrators are given rules (by directors, and by social pressures of other illustrators), whereas artists give themselves rules. this is a key distinction to make between the creative processes of each maker.